Device having an electric discharge tube comprising a field emission cathode, and discharge tube destined for such a device

ABSTRACT

In a cold cathode consisting of a layer of aluminum oxide between a plate and a grid, the aluminum oxide is formed on aluminum as a porous oxide layer. The meshes of the grid are so fine that the grid is attracted against the oxide due to the voltage difference with the plate.

United States Patent Jansen et a1.

[ 51 Aug.-29, 1972 DEVICE HAVING AN ELECTRIC DISCHARGE TUBE COMPRISING AFIELD EMISSION CATHODE, AND

DISCHARGE TUBE DESTINED FOR SUCH A DEVICE Inventors: Constant GiisbertusJohannes Jansen; Klaas Compaan; Albertus Venema, all of Emmasingel,Eindhoven, Netherlands U.S. Philips York, NY.

Filed: Sept. 14, 1970 Appl. No.: 71,806

Assignee: Corporation, New

Foreign Application Priority Data Sept. 18, 1969 Netherlands ..6914205US. Cl. ..313/348, 313/268, 313/337 Int. Cl ..H0lj 1/46, HOlj 1/52, HOlj17/04 Field of Search ..3l3/348, 268, 337

[56] References Cited UNITED STATES PATENTS 3,363,136 1/1968 Pakswer..313/268 1,618,499 2/ 1927 White ..313/268 UX 3,154,711 10/1964 Beggs..3l3/337 X FOREIGN PATENTS OR APPLICATIONS 1,359,344 3/1964 France..3l3/268 Germany ..3l3/268 Primary Examiner-David Schonberg AssistantExaminer-Toby H. Kusmer Attorney-Frank R. Trifari ABSTRACT In a coldcathode consisting of a layer of aluminum oxide between a plate and agrid, the aluminum oxide is formed on aluminum as a porous oxide layer.The meshes of the grid are so fine that the grid is attracted againstthe oxide due to the voltage difference with the plate.

4Claims,2DrawingFigures IXVEXTORS CONSTANT G.J. JANSEN KLAAS MPAANALBERT S VE NE MA AGENT DEVICE HAVING AN ELECTRIC DISCHARGE TE(IQMPRISING A U) CATHODEDESC GETUBED FOR SUCH A DEVICE The inventionrelates to a device having an electric discharge tube comprising a fieldemission cathode consisting of a layer of aluminum oxide on which aconductive metal layer is present on one side and a metal grid which hasa positive potential relative to the conductive metal layer is presenton the other side. The invention furthermore relates to an electricdischarge tube destined for such a device.

The aluminum oxide in the cathode described in the German patentspecification 746,016 consists of a comparatively thick sintered mass.The metal layer is provided on the aluminum oxide as a metallization Themetal grid is preferably pressed in the aluminum oxide layer. Theemission current is assumed to consist of electrons which move from theinterior of the aluminum oxide to the apertures in the metal grid andthere either emanate themselves, or liberate secondary electrons fromthe edges of the grid. Once the emission has started, it is notnecessary to apply an external potential difference between the twoelectrodes at the aluminum oxide, since as a result of the emission thepotential difference is automatically maintained. However, this appliesonly if the cathode goes on supplying current. In so far as it is known,the cathode described has not resulted in practical applications anddata on the emission density to be achieved with the cathode and thevalue of the applied voltages are not known either.

From French patent specification 1,359,344 a cold cathode is known whichconsists of an aluminum plate on which a layer of aluminum oxide with athickness of from 100 to 300 AU is formed by vapor-deposition or byanodic oxidition. On this layer is present a layer of a metal, such asgold, platinum or palladium, obtained by vapor-deposition, which layerin turn is covered with a thick porous layer of magnesium oxide whichhas likewise been obtained by vapor-deposition. The thickness of thislayer is in the order of 50 pm. The cathode requires a voltage ofapproximately volts across the layer of aluminum oxide.

In the cathode known from French patent specification 844,616 a layer ofa material having a low work function, an alkali metal or alkaline earthmetal is present between a thin aluminum oxide layer and the support.The aluminum oxide is covered by a gauze, or by a very thin metal layerwhich is permeable to electrons. With a voltage of approximately 1 voltacross the aluminum oxide, currents in the order of l A could be reachedwith a value of the cathode which is not stated. In order to stimulatethe emission, the cathode can be slightly heated or be exposed toinfrared radiation. The field emission occurring in the cathode is saidto be obtained as a result of all these measures at field strengths of10 V/cm a factor 100 to 1,000 lower than normal. Practical applicationsof said cathode are not known.

It is the object of the invention to provide a device of the typementioned in the preamble having a simple and rigid construction.

According to the invention, in a device having an electric dischargetube comprising a field emission cathode consisting of a layer ofaluminum oxide on which a conductive metal layer is present on one sideand a metal grid which has a positive potential relative to theconductive metal layer is present on the other side, the aluminum oxidelayer consists of a porous oxide layer having a thickness of from a fewpm to a few tens um formed on an aluminum support by anodic oxidation.

Such porous aluminum oxide layers which have long been known already(see A.J. Decker and W.Ch. van Geel, Philips Res. 2 313-319, 1947) havea substantially hexagonal structure in which cavities and pillarlikeparticles of aluminum oxide, both with a diameter of a few hundred of AUdirected at right angles to the surface, seem to alternate. The cavitiesand pillars of oxide are separated from the underlying aluminum by aclosed layer of aluminum oxide the thickness of which expressed in tensof AU is approximately equal to the voltage expressed in volts appliedin the electrolytic formation of the layer. As an electrolyte in theformation of the layer is used oxalic acid or another acid in whichexclusively the formation of aluminum oxide occurs.

In connection with the thickness of the porous oxide layer, the requiredpotential difference across the layer is in the order of 50 volts orhigher. However, this involves that if the metal grid has a sufficientlylow thickness, wire thickness 50 um and less, the gauze is attracted,throughout the surface of the cathode, by the electrostatic force. Theadvantage of this with respect to a more rigid grid of thicker wireswhich is held only by mechanical force is that it better engages thesurface of the cathode, which promotes the uniformity of the emissionacross the surface.

In order that the invention may be readily carried into efiect, it willnow be described in greater detail, by way of example, with reference tothe accompanying drawing, in which FIG. I is a diagrammaticrepresentation on a strongly exaggerated scale of the electrode systemof an electric discharge tube according to the invention with theassociated electric connections and FIG. 2 shows a few current-voltagecharacteristics.

Reference numeral 1 in FIG. 1 denotes a strip of aluminum which has beenbent at right angles and has an upper surface area of 0.3 sq.cm. Aporous oxide layer 2, thickness 5 pm, in which cavities are present atright angles to the surface, is present on the aluminum, the cavitiesand the intermediate oxide having transverse dimensions of from 200 to500 AU. Below said layer is a thin layer of aluminum oxide having athickness of approximately AU. A copper gauze 3, mesh width 40 um, wirethickness 7 pm, is present on the aluminum oxide 2. The anode isconstituted by a nickel plate 4 at a distance of 2 mm from the gauze 3.The electric connection of the aluminum plate, the gauze and the nickelanode are denoted by 5, 6 and 7. The current limiting resistors 8 and 9,respectively, each 1 Mil, are included in the connection leads to 5 and7.

In FIG. 2 the voltage difference between the connection terminals 6 and'7 in volts is plotted on the horizontal axis, and the electron currentin A which flows to the anode 4 is plotted on the vertical axis, curve Aapplying to the case in which the voltage between 5 and 6 is volts andcurve B applying to the case in which said voltage is volts. Thecurrents in said circuit are 2.0 and 12.5 A respectively.

What is claimed is:

1. A device having an electric discharge tube comprising a fieldemission cathode consisting of a layer of aluminum oxide on which aconductive metal layer is present on one side and on which a metal gridwhich has a positive potential relative to the conductive metal layer ispresent on the other side, said aluminum oxide layer consisting of aporous oxide layer formed on an aluminum support by anodic oxidation.

2. A device as claimed in claim 1, wherein the potential differenceacross the aluminum oxide layer is so large that the metal grid isattracted against the aluminum oxide layer by the electrostatic force.

3. A device as claimed in claim 2, wherein the poten-

2. A device as claimed in claim 1, wherein the potential differenceacross the aluminum oxide layer is so large that the metal grid isattracted against the aluminum oxide layer by the electrostatic force.3. A device as claimed in claim 2, wherein the potential differenceacross the aluminum oxide layer is more than 50 volts.
 4. An electricdischarge tube destined for a device as claimed in claim 1, wherein theporous aluminum oxide layer has a hexagonal structure in which cavitiesat right angles to the surface alternate with intermediate material,both with transverse dimensions in the order of magnitude of hundreds ofAU and the cavities and the intermediate material are separated from theunderlying aluminum by a closed layer of aluminum oxide having athickness in the order of magnitude of hundreds of A-units.